Not the Depp of Pirates of the Caribbean, mind you. More the Johnny of the Chocolate Factory or Alice in Wonderland. Willy Wonka Depp. Mad Hatter Depp.

It's because what's in front of me is just so fantastical. This is the underwater world as reimagined by director Tim Burton. It just needs some kooky Johnny Depp mer-creature to swim into frame.

​The majesty of the Great Barrier Reef has to be seen to be believed. Photo: Supplied

It's like we've hit some Great Barrier Reef sweet spot: the water crystal-clear and bath-water warm, the sky cerulean and unencumbered by clouds.

I upend and push down into the depths, towards the bright white sand between the coral columns and canyons, and then just hang there, in the almost-silence, in the moments between one breath and the next, and … marvel.

I think of Depp and, oddly, of my 86-year-old mother at home in London in her small housing commission flat and how I want to hold her hand and show her this and say "look, look at all this beauty". And I think of how she never will, no matter how many David Attenborough documentaries she watches. Because nothing, nothing, prepares you for the real thing.

And I know that later, when I sit down to make notes about what I've seen, I will fall into despair at the impossibility of using mere words to describe the experience. I'll also be pissed off that I didn't bring an underwater camera.

***

Right, you all know the drill. It's time for the stats. This is the Great Barrier Reef, the world's largest coral reef system. It is a gazillion kilometres long, a pulsating, living, breathing natural behemoth consisting of a quadrillion tiny islands, islets, bommies, reefs and God only knows what else. The reefs are built by minute organisms known as coral polyps. The jury's still out but there are about infinity squared plus one of these things.

Yup, it's very big. And very small at the same time.

And according to scientists the whole thing's going to hell in bleached handbasket. It's such a powerful image that, on the plane to Cairns, I find myself wondering what awaited; paradise or sub-aqua dystopia? Mardi gras or the inside of Donald Trump's head?

Luckily, the people at Coral Expeditions know where to find the good bits. Ribbon reef #9 being a case in point.

A room on board. Photo: Supplied

I am suspended in the warm water, in a narrow canyon between three or four bommies (dome-like standalone conglomerates of coral). Above, the sun plays games with the surface of the water and dapples everything below with a sparkling, luminescent interplay of light and shade.

The coral here is like some phantasmagorical garden city on a far-off planet, its organic architecture all at once familiar and foreign. There are soaring pillars, bright blooms of arboreal staghorn coral and flat, veined discs like outstretched palms or helicopter landing pads. Below these hunker the bulbous forms of brain coral, like the rooftops of concert halls designed by Guggenheim on acid. And everywhere, in a beautifully otherworldly touch, there are the crumpled blue lips of burrowing clams smiling back at you. Dali would have loved it.

And that's just the hard stuff. Between all this there are patches of soft coral slowly swaying to music only they can hear. It's as if a corps of porky ballet dancers has squeezed through here and left swatches of torn but handsomely hued tulle behind on the sharp edges.

Fish life and coral on Flynn Reef off the coast of Cairns.

And, oh, the colours … it's like one of those tricked-up photographs that NASA releases of far-flung galaxies. There are iridescent reds and brilliant blues and perfect pinks and every shade of pretty much every other colour under the sun. There's even a chocolate brown with turquoise spots.

And between all this? Fish. Fish of all shapes and sizes and colours and disposition. There are fish that seem to change colour from brown to blue as they move; tiny electric blue fish with even brighter blue fluorescent halos; clouds of black and white zebra fish (that's how I think of them anyway) dancing in the current; fish like splashes of paint, fish like dropped scoops of tutti-frutti ice-cream, fish like potted rainbows.

There are starfish bursting with colour, fish with long snouts, fish with belligerent faces and boxy bodies, fish so slim you could slip them under a closed door, fish so small they'd fit up a nostril.

There are hump-headed Maori wrasse, blue-and-yellow striped surgeon fish, angel fish and dozens of uber-elegant Moorish Idols (the Audrey Hepburns of the reef, if you ask me). It's hard to know where to look first.

Later, floating on the outer edges of the reef where the coral drops away into deeper water I spy giant trevally and a couple of slender white-tipped reef sharks slinking silkily along the sandy bottom. My favourite moment of the day is the squadron of seven pale squid which sails past in a line, each member equidistant from its companion, their little wings flapping away like tiny propellers.

***

The Coral Expeditions cruise is divided up into three and four-day sections from Cairns. I'm on for the full seven-day run but some people only do one or the other. This is lucky because for the first three days or so the weather isn't kind to us.

We start from Cairns and head south to Fitzroy Island on squally water, grey skies and intermittent rain. The snorkelling here is not great. There are fish, yes, but the water is murky thanks to the burgeoning storm and the coral is white and grey and beige, like the million blanched bones of some chicken genocide.

The next day we cruise through Hinchinbrook Channel and take the glass-bottomed boat out for a rain-spattered mooch around the mangroves with guide Dani. In the afternoon, after a barbecue lunch and a reef creature feature lecture in the lounge, we trudge wetly to the lookout on Dunk Island and wonder where the famed Queensland sun has gone.

It returns, thankfully, with a vengeance the next day at Nathan and Coates reefs, and the day after at Sudbury Cay, a sandbank that pokes its head surreally out of the ocean in the middle of nowhere. From this narrow, low-slung sliver of sand we kayak and snorkel until called back to the boat.

It's the beginning of a golden few days of blue skies and sparklingly clear waters in which we visit Lizard Island, Cooktown, Ribbon Reef #3, Escape Reef and Ribbon Reef #9. We snorkel for hours, not wanting to come back in. We take glass-bottomed boat tours and watch peach-coloured sunsets with the satisfied bone weariness of people who have exercised hard, seen something special and gone to places that the general run of reef day trippers never get to see.

Really. Go now. While it's still there.

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TOUR

Three, four and seven-night cruises depart from Cairns year-round, with a seven-night cruise starting at $3395, including meals, welcome drinks, presentations by marine biologists, access to islands and marine parks, excursions, snorkelling equipment, flotation vests and wetsuits. Stinger suits are available for an extra charge. See www.coralexpeditions.com